1. Field of the Invention
The invention relates to a protection circuit for protecting a transistor from overloading. Such a protection circuit can, for example, be used in integrated power amplifiers for audio equipment. The output transistors of such power amplifiers must be operated within the Safe Operating Area Rating (SOAR) to prevent the transistors from being damaged by overloading.
2. Description of the Prior Art
Such a protection circuit is, for example, used in the integrated audio amplifier TDA 1520, described in Philips Data-Handbook: Integrated Circuits, January 1983. Therein, the first means comprises a first part resistor included in the collector or the emitter lead of the transistor. A first signal is derived from the voltage across this resistor, which signal consequently is proportional to the current through the transistor. The second means comprise a voltage divider formed by second and third resistors and arranged between the collector and the emitter of the transistor. A second signal is derived from the voltage across the second resistor, which second signal is consequently proportional to the collector-emitter voltage of the transistor. Above a given collector-emitter voltage, the what is referred to as knee voltage, the voltage across the second resistor is limited by a Zener diode arranged in parallel therewith, so that the second signal has a constant value above the knee voltage.
The first and second signals are added together whereafter this sum signal is compared with a threshold value. If the sum signal becomes greater than the threshold value, then a transistor is rendered conductive which diverts base current from the output transistor and consequently limits the current through the output transistor.
From the fact that when the protection becomes operative the sum of the first and second signals is controlled in such manner that this sum is equal to the threshold value, it follows that the limited value of the current through the output transistor is equal to the difference between a fixed current which is proportional to the threshold value and a variable current which is proportional to the collector-emitter voltage of the transistor.
With very small collector-emitter voltages the second signal is so small as to be disregarded. In that case the protection becomes operative if the first signal, which is proportional to the collector current, becomes greater than the threshold value. The implication is that the maximum current through the output transistor is proportional to the threshold value.
Since the voltage across the second resistor is limited by the Zener diode, a small residual current may continue to flow through the output transistor in the event of collector-emitter voltages above the knee voltage. This residual current is necessary to ensure that the circuit can operate correctly upon application of the supply voltage, as the load, for example a loudspeaker, is coupled to the output of the output transistor via a capacitor. Upon switch-on a certain current is required to charge this capacitor. The residual current is also required to prevent distortion of the output signal in the event of a large voltage swing. This residual current is equal to the difference between the maximum current through the transistor, which is proportional to the threshold value, and a fixed current which is proportional to the knee voltage.
Now the residual current is much smaller, for example by a factor of 20, than the maximum current through the output transistor. Consequently, the residual current is formed by the difference between two currents which are each much larger than the residual current itself, in the present example 20 and 19 times larger, respectively. As a result thereof, the residual current is highly dependent on variations in these currents. A variation of, for example, 5% in one of the currents causes a variation of 100% in the residual current. Precisely in the area above the knee voltage with high collector-emitter voltages an excessive residual current may easily result in the output transistor being damaged. Too small a residual current has for its result that the output signal is distorted in the event of a large voltage swing or that the circuit cannot build-up to proper operation upon application of the supply voltage.